Monday, November 8, 2010

Learning to Drool... Part 1

This series of posts will be about me getting to grips with JBoss Drools. The reasoning behind it is: SAP bought out my company's current rules engine and Drools is one alternative we will be looking into as soon as someone has the skills to get a proof of concept up.

Although there seems to be a fair amount of documentation, I always find it helps having walked through examples, which is what I am going to do here.Drools on first glance can be quite daunting, it is made up of :

Drools Expert (rule engine)
Being a developer this is where I will begin, the actual rules and implementation of them.

The other parts I'll get to later are:
Drools Guvnor (BRMS/BPMS)
Drools Flow (process/workflow)
Drools Fusion (event processing/temporal reasoning)
Drools Planner (automated planning)

So to begin.
For part 1, I just want to get my feet wet, I download only the Eclipse plugin and the binaries

Need to install the Eclipse plugin, used the update site. Unzip the binaries to a directory and withing the Eclipse plugin settings point to that directory.

The eclipse plugin will allow you to create a Drools Project and that includes the "Drools Library", but if you are using maven you need to point to the JBoss release repo for the Drools dependencies, The KnowledgeRuntimeLoggerFactory needs XStream which you can just get from the standard maven repo. Below is my POM:

To add a rule, on src/main/rules, Right click -> New -> Other... Drools/Rule Resource, be sure to choose the "individual rule". This leaves you with an empty drl file:

To understand and use the rule language, I read the Drools Documentation and the examples project.
What actually took me a little to grasp, was the basic syntax and how objects were handled in the rules and I did struggle to find anything that actually explains it simply so I will give it a shot.

A note on variable names.. they do not need to have the '$' but it was used in the example and without it quickly gets quite confusing.

Now to step through the parts of a rule:

The package, and import keyword are self explanatory, what happens after the when is not.
What is happening in "$vo : RoolVO( stringValue == "Learning to drool", $booleanVal : booleanValue )" broken down actually is:

stringValue == "Learning to drool" - This is a constraint that allows us to find all RoolVO objects known to the knowledge base that have the getStringValue() value equals to "Learning to drool". If there are multiple instances of RoolVO that comply we will run this rule more than once, these are also referred to as matched objects. You can also have multiple constraints separated by a ",".

$booleanVal : booleanValue - We are declaring a new local variable of type boolean called $booleanVal and gets it value from isBooleanValue.

$vo : RoolVO - We are declaring a new local variable of type RoolVO called $vo.

The next line:
"eval( $booleanVal )" - Evaluates the boolean variable, this needs to evaluate to true for the 'then' part of the rule to be called.
System.out.println( "First Rule" ); - Standard sys out.
$vo.setStringValue("Done."); - Sets the String value on the current RoolVO object that matched the constraints to Done.

The main classes / interfaces needed for a basic rule execution seem to be the following:
org.drools.KnowledgeBase and it's factory
This is the repository of all the relevant knowledge definitions; it contains rules, processes, functions, type models.

org.drools.builder.KnowledgeBuilder and it's factory org.drools.builder.KnowledgeBuilderFactory:
Transforms / parses a source file (.drl, .xsl) into a KnowledgePackage that a KnowledgeBase can understand.

StatefulKnowledgeSession created by the KnowledgeBase .newStatefulKnowledgeSession();
This session is used to communicate with the actual rules engine.

To quote the drools JavaDocs:
StatefulKnowledgeSession is the most common way to interact with a rules engine. A StatefulKnowledgeSession allows the application to establish an iterative conversation with the engine, where the reasoning process may be triggered multiple times for the same set of data.

I wrote a simple test for the rule I described earlier.

To be honest, Drools is not nearly as intuitive as Quickrules was from what I have seen in this first task. However, in Part 2, I will start looking at using the rule flows, decision table functionality as well as the guided rules:

Those in partnership with the DSL (Domain Specific Language) construct hopefully do allow for more intuitive rules creation.


  1. Promises to be an interesting series. I've wanted to look into Drools a couple of times, but never got to it. This was probably because it seemed to have a pretty steep learning curve and had too many bits and pieces.

    How will you handle migration from Quickrules, is there support for importing from other rules engines?

    By the way I thought I'd just let you know, it doesn't look like your code snippets are showing up in the RSS feed. I can see the code in the XML but when rendered in HTML it's blank.

  2. yeah, I felt exactly the same way, been wanting to start learning for ages, so I "forced" myself by publicly declaring it in a previous post. :)

    There isn't any I have seen, all the quickrules rule info is in a DB... will have to write a custom conversion kit.

    Yeah, I know SyntaxHighlighter java script doesn't work in RSS, you know of something else I can use that will? (besides images).

  3. Aye, migration from QuickRules will be painful. There are no standard structures for rule formats currently (JSR94 looks at execution as opposed to format - well at least the last time I looked at it). But Brian is volunteering to look at some kind of conversion tool.

    I reckon we will end up migration rulesets across and doing a bit of rules refactoring at the same time.

  4. Note that eval() is not indexed, so if isBooleanValue() is constant or only changed using a modify statement then you should test its value inside a conditional expression instead.

  5. Thanks, yeah I figured as much... this was also just to show another variable creation... but will keep it in mind.

  6. Think of your "when" stuff as where clauses in a select statement.

    So RoolVO( stringValue == "Learning to drool" ) would be kind of like:

    SELECT * FROM RoolVO WHERE RoolVO.stringValue = 'Learning to drool';

    You'll find if you start thinking like this you'll start building more Drools-friendly rule constructs. I hope this helps.


  7. Thanks for the comment Laird... that is actually a very nice way to think about it.

  8. Hi, i like your drools-blog :) Will there be a part 3 with jBPM or other?

    What is your conclusion. Can Drools replace Yasu-Quickrules? How do you implement base rules in Drools?

  9. very knowledgeable post.! Good to learn . It would be helpful for me if you could please provide example of : how to use Excel Spread sheet with drools, like dynamic calculations depending on the requirement..??

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